In micro machining operations such as in the range below 0.040", for example, it is essential that both tool infeed pressure and tool infeed depth be carefully controlled and that infeed pressure be applied in such fashion that no lateral force components are imparted to the tool. This because of the inherent fragility of micro tools as a function of their small diameters, many of which are barely perceptible to the human eye.
The state of the art insofar as micro tool infeed pressure application is concerned is exemplified by the cross arm pivot mounted drill feed mechanism manufactured by National Jet Company, Cumberland, Maryland, for its Model 1M micro drilling machine as illustrated and described in National Jet Company Technical Bulletin No. M1-168. This infeed mechanism is normally balanced for zero torque exertion with a drill travel and sensitivity ratio producing a 20-fold touch sensitivity amplification. Such extreme sensitivity in the control of infeed pressure is, of course, critical in micro drilling operations since even if the drill is maintained in pure compression it is subject to fracture if too great an infeed pressure is applied.
In similar fashion, depth gauging is of critical importance in micro drilling to prevent excessive drill heat, permit chip removal and avoid tool breakage by the inattentive operator. Such a depthing device allowing operator control of hole depthing advance from 0.0004" to 0.005" is manufactured by National Jet Company under Model No. F as described in National Jet Company Accessory Bulletin No. 0 DD-1167.
In the case of macro machining operations, the foregoing considerations of infeed pressure and depthing control are of lesser concern from the standpoint of tool breakage since the larger tool diameters, per se, provide inherent strength, making the tool more forgiving of that rough usage typically associated with macro machining such as, for example, the application of excessive infeed pressures.
Consequently, that accuracy of control necessarily associated with micro machining has not been applied to macro machining operations even though the corollary benefits of such control are significant. Exemplary is the accurate blind hole bottoming that is only possible in the absence of reactive machine spring back in response to infeed pressures, cooler running tools with consequent life extension and faster machining operations since excessive tool wear is one function of excessive infeed pressures.
While the instant invention is concerned with the convertibility of a single machine for both micro and macro machining operations which may include, inter alia, drilling, reaming, broaching and boring; the infeed control criteria that apply in micro machining operations are inapplicable for macro machining. In the case, for example, of the cross arm infeed mechanism used for micro drilling the relatively great length of the cross arm, infeed lever and connecting rod that make possible the sensitivity amplification necessary for micro drilling provides a long reaction lever arm which, when the necessarily greater infeed pressures are applied in macro drilling, results in a substantial "spring back" in reaction to drill infeed. This "spring back" results, inter alia, in inaccurate depthing. In further explanation; that path, through the machine frame, along which the reaction to tool infeed against the workpiece travels back to the point of input pressure application is that path along which spring back occurs. As is obvious, the extent of spring back is a direct function of input pressure, reaction path length, configuration of path length and mass and material through which the reaction path length extends. Thus if, for example, it is desired to advance a macro drilling tool a distance X into a workpiece and the depthing gauge against which pressure application will be directly applied is set for a depthing distance X, as the tool engages the workpiece the resistance encountered produces a reactive "spring back" through the machine frame to the point of pressure application of magnitude Y as considered linearly along the overall path length. Under such circumstances the actual hole advance will be X-Y rather than the desired hole advance X. To reiterate, this "spring back" just described is not a significant factor affecting micro machining operations because there are other, more limiting, parameters that make it necessary to exert such small infeed pressures as to render the spring back factor negligible, viz., considerations of tool breakage. If, on the other hand, such small infeed pressures were used in connection with macro machining, machining time would be prohibitive.
Inaccurate blind hold depthing is a major cause for part rejection in two large fields of machining activity; diesel nozzle and spinnerette manufacture. Thus, the macro drilled blind sac hole in a diesel nozzle and the counterbored blind hole depth in spinnerettes must be held to closer tolerances than are permitted by that inherent spring back in conventional drilling machines applying maximum rated infeed pressure. Consequently, it is necessary to use less than desirable infeed pressure as the desired hole bottom is approached to remain within tolerances. The result is increased machining time. Of equal consideration is the inattentive operator who continues to apply maximum infeed pressure as hole bottom is approached resulting in a blind hole to less than spec depth and/or the operator who does not have the "feel" for the lesser pressure application. A typical diesel nozzle sac hole is 0.118" in diameter while a typical spinnerette counterbore is 0.125" in diameter, both of which size macro drills have the inherent strength to produce the spring back herein referred to at rated infeed pressure applications for such sized tools.
In addition to workpiece inaccuracy, spring back produces machine fatigue in those movable and interfitted parts lying along the spring back path. The "feel" of the occurring spring back is a "spongy" feel in the equipment and the relative movement produces machine frame wear particularly adjacent working bushings and moving parts.